Hydantoin-formaldehyde as binder material for photoconductive substances

ABSTRACT

ELECTRO-PHOTOGRAPHIC RECORDING MATERIAL WHEREIN THE PHOTOCONDUCTIVE SUBSTANCE, PREFERABLY PHOTOCONDUCTIVE ZINC OXIDE, IS DISPERSED IN A BINDER CONTAINING AT LEAST 60% BY WEIGHT OF A HYDANTOIN-FORMALDEHYDE CONDENSATION POLYMER AND CHARACTERIZED BY RAPID REGAIN OF ITS DARK-RESISTIVITY SO AS TO BE SPECIALLY ADAPTED FOR RAPID REPETITIVE CHARGING AND EXPOSURE.

United States Patent US. Cl. 961.5 7 Claims ABSTRACT OF THE DISCLOSURE Electro-photographic recording material wherein the photoconductive substance, preferably photoconductive zinc oxide, is dispersed in a binder containing at least 60% by weight of a hydantoin-formaldehyde condensation polymer and characterized by rapid regain of its dark-resistivity so as to be specially adapted for rapid repetitive charging and exposure.

This invention relates to recording members suited for used in electrophotography.

Photoconductive binder type coatings prepared by incorporating a finely divided photoconductor in a binder are well known.

In such coatings, various inorganic as well as organic substances can be incorporated as photoconductive substances, e.g. sulphur, selenium, the oxides, sulphides and selenides of zinc, cadmium, titanium, mercury, antimony, bismuth and lead, anthracene, anthraquinone and more recently discovered organic monomeric and polymeric organic photoconductors, such as e.g. those described in the Belgian patent specifications 587,300, the UK. patent specifications 964,871; 964,873; 964,874; 964,875; 964,876; 964,877; 964,879; 970,937; 980,879 and 980,880 and in the German patent specification 1,058,836 and in the Canadian patent specification 568,707.

The use of photoconductive substances combined with insulating binder agents is known among others from the US. patent specifications 2,197,552; 2,297,691; 2,485,589; 2,551,582 and 2,599,542 and from the UK. patent specifications 566,278; 693,112 and 700,502.

C. 1. Young and H. G. Greig R.C.A. Rev. 15 (1954) 469, describe an electrophotographic material containing a photoconductive layer comprising a specially prepared photoconductive zinc oxide in an insulating resin binder. Said layer is coated on a sufiiciently conductive paper base, e.g. a clay-coated paper, from a paint-like coating mixture by conventional paper-coating techniques. Since the zinc oxide has a brilliant white colour it forms an excellent base for contrasting with the developer materials applied to it.

According to existing techniques, the photoconductive binder coatings are prepared by dispersing or dissolving the photoconductive substances in an organic solution of an insulating binder, and by applying them as such in the form of a layer to an apporpriate relatively conductive support.

Now we have found that a photoconductive recording element having interesting properties for recording electromagnetic radiation images can be prepared by a method comprising the step of dispersing an inorganic photoconductive substance e.g. photoconductive zinc oxide in a hydantoin-formaldehyde condensate, the structural units of which correspond to the following formula:

wherein each of R and R represent hydrogen or a lower alkyl radical (C1C7).

The intrinsic viscosities of the said polymers preferably ranges between 0.02 and 0.20, measured in a mixture of water and technical concentrated ammonia (25%) in a ratio 9: 1.

A recording layer coated from the above-mentioned dispersion is characterized by a quick and high chargeability and a high sensitivity even without spectral sensitizing agent(s).

In preparing the said recording layer the hydantoinformaldehyde resin is preferably applied from an aqueous medium which offers all the advantages associated with the absence of organic solvents in the coating composition and requires no special coating techniques.

The coating medium is preferably made alkaline and for that purpose preferably a volatile base (boiling point below C.) is used e.g. ammonia. It was experimentally stated that a coating composition with a pH above 7 yields recording layers with a relatively soft gradation; see therefore Example 3.

The recording element prepared according to the present invention possesses the property of regaining rapidly, after charging and exposure, its original dark-resistivity so that this element is very suited for multicolour reproduction wherein generally at least two successive chargings and exposures of the element are required.

The recording element prepared according to the present invention is flexible, possessess a very good mechanical strength and shows a sufficient adhesion to a paper support as well as to a metal support, e.g., an aluminium support.

The resistance of the photoconductive coatings according to the present invention to non-polar organic solvents is high, so that by virtue of this property the recording layers are very well suited for electrophotographic reproduction techniques wherein an insulating hydrocarbon liquid containing charged pigment particles is used (electrophoretic development).

The order of mixing steps for the constituents to form the coating is not important. The kind of photoconductor does not play a part, e.g. any known inorganic photoconductor may be used.

When preparing the photoconductive coating from an aqueous medium with photoconductive substances which are not water-soluble the use of a good dispersing agent is necessary. For that purpose the dispersing agents described agents described in the publ. Dutch pat. application 6712156 are advantageously used.

The quantitative ratios of the photoconductive substances to the binding agents, wherein the hydantoinformaldehyde resin preferably stands for at least 60% by weight of the total binder content, may vary between wide limits. Other binding agents applicable from an aqueous alkaline medium are, e.g., those described in the United Kingdom patent specifications 888,371; 932,730; 938,366 and 881,613, the publ. Dutch patent applications 6608815 and 6608814 and in US. patent specification 2,959,481.

It is preferred to apply the photoconductive substance in a ratio of 1 part by weight to 0.2 to 1.5 parts by weight of the total content of binder. The thickness of the photoconductive layer may vary between wide limits according to the requirements of each case. Good recording and reproduction results are obtained with electrophotographic layers with 15 to 25 g. of photoconductive substance per square meter. Too thin layers possess an insufiicient insulating power, whereas too thick layers possess undesirable mechanical properties.

The photoconductive recording layers prepared according to the present invention may contain, in addition to the photoconductive substance(s) and hydantoin-formaldehyde binder, acid compounds for increasing the darkresistivity as described e.g. in UK. patent specification 1,020,504 and Belgian patent specification 612,102 and additives known in coating techniques, e.g., dispersing agents and compounds of influencing the gloss and/or the viscosity, and compounds which counteract ageing and/or oxidation of the layers, or which influence the thermal stability of the layers. When selecting any additives preference is given to those additives, which least reduce the dark-resistivity of the photoconductive layer.

The spectral sensitization of the materials prepared according to the present invention is advantageously performed by means of the sensitizing dyes mentioned in the U.K. patent specification 1,020,504, and in the publ. Dutch patent application 6704706 and pub]. Dutch patent application 6717400. The sensitizing dyes are preferably applied according to a sensitizing techique described in the publ. Dutch patent application 6704768.

The photoconductive coating composition according to the present invention may be coated on a support by a known coating technique e.g. by spraying, whirling, dipcoating or by a coating technique wherein use is made of a doctor blade. The supports or base materials are chosen in view of the particular charging, exposing, recording, developing and/ or transfer technique wherein the recording material is to be used. Any support suited for use in electrophotographic materials is here included. As examples thereof are mentioned metal sheets, resin layers coated with a conductive interlayer or intersheet and a paper sheet either or not coated with a conductive pigment layer or laminated with a conductive metal foil. As paper supports having no pre-coat, preferably glassine type paper sheets are used.

In electrophotographic recording techniques wherein the photoconductive layer is electrostatically charged, the support has preferably an electric volume resistivity considerably lower than that of the recording layer, preferably has a volume resistivity of at least times less than that of the recording layer. Suitable supports are described, e.g., in UK. patent specifications 1,020,504 and 995,491 and in US. patent specification 3,008,825.

Recording elements according to the present invention are also suited for recording light images without it being necessary to non-differentially charge the recording element beforehand, which means that sufliciently high concentrated technical ammonia (9:1) as dispersing agent and 0.62 cc. of acid butylphosphate as a 10% by weight solution in ethanol. The obtained slurry was then sub jected for 15 min. to high speed stirring with a Kothofi' mixing sirene. The pH of the dispersion was measured and the value 6.9 noted.

The zinc oxide dispersion was coated onto a glassine type paper at a rate of 20 g. per sq. m., dried and stored in the dark for 24 hours.

After charging the dried layer with a negative corona of 6000 volt it was exposed through a step wedge with constant 0.1 by means of a light bulb (colour temperature 2750 K.) having a luminous intensity of 2400 lux.

The exposed layer was developed with an electrophoretic developer. The maximum density obtained was 0.8 and the minimum density 0.18. The number of blackened steps between the maximum density and minimum density being a measure for the gradation was 3. The number of steps which were not covered with developing substances being a measure of the light-sensitivity of the recording layer was 7.

EXAMPLE 2 Example 1 was repeated but instead of the dimethylhydantoin-formaldehyde resin at same amount of a hydantom-formaldehyde resin having an intrinsic viscosity of 0.08 measured in a mixture of Water and technical concentrated ammonia (25%) in a ratio 9:1 was used.

The results were the same of those obtained according to the procedure of Example 1.

EXAMPLE 3 Example 1 was repeated but various amounts of ammonia used in order to raise the pH of the dispersion values between 6.9 and 11.1. The highest amount of technical ammonia (25% by weight) necessary was 6 cc. The different compositions were separately coated and developed in the same manner as in Example 1.

The photographic results are listed in the following table. The sensitivity is expressed by the number of nonblackened steps; the higher that number the more sensitive the recording layer. The gradation is expressed by the number of blackened steps between maximum and minimum optical density; the higher that number the softer the gradation.

pH of the dispersion Sensitivity 7 7 7 7 6 7 6 6 6 6 6 Gradation 3 3 3 3 4 3 5 9 6 8 7 Max. optical density 0.83 0. 67 0. 73 0. 67 0. 79 0. 89 0. 69 0. 73 0. 77 0. 76 0. 80 Min. optical density 0. 18 0. 18 0. 19 0. 16 0.19 0. 20 0.19 0. 17 0.17 0. l6 0. 18

EXAMPLE 1 To a solution of 12 g. of dimethylhydantoin-formaldehyde resin (the intrinsic viscosity of said resin measured in a mixture of water and technical concentrated ammonia (25%) in a ratio 9:1, is 0.04) in cc. of water, 15 g. of photoconductive zinc oxide (French Process type) were added and subsequently during slowly stirring 0.75 cc. of a 10% by weight solution of copoly(maleic anhydride/N-vinylpyrrolidone) 40) in a mixture of water/ What we claim is:

1. A recording material, which comprises a support and a recording layer containing a photo-conductive substance dispersed in a binder containing at least 60% by weight of a hydantoin-formaldehyde condensate the structural units of which correspond to the following formula:

wherein each of R and R is hydrogen or lower alkyl radical of 1-5 carbon atoms.

2. A recording material according to claim 1, wherein the photoconductive substance is photoconductive zinc oxide.

3. A recording material according to claim 1, wherein the support is a glassine type paper support.

6 4. A recording material according to claim 1, wherein References Cited spectrally sensitizing dyes are present in the recording UNITED STATES PATENTS layer.

5. A recording material according to claim 1, wherein 2155863 4/1939 Jacobson 260 309 X the photoconduc tive substance and binder are applied from 5 an aqueous medium to the support.

6. A recording material according to claim 5, wherein GEORGE LESMES, Primary EXamineI the Said medium iS alkall'lle- J. R. MILLER, Assistant Examiner 7. A recording material according to claim 5, wherein 10 the said medium is made alkaline by means of a volatile base. 96l.7, 1.8 

